This sounded terrible. I suspect the filters aren't great and caused phase distortion.

I returned to my original setup of HEDD192 directly into T16s and I'm running the Emes sub from another set of D/As.

I would ideally like to "Y-split" the Cranesong's outputs to connect both the T16s and Emes sub, but I'm concerned about impedance mismatches and the associated frequency response issues.

The Emes Amber sub has an input impedance quoted as 10kohm and the ATC T16s are quoted as >10kohm. I couldn't find any details on the Cranesong HEDD192's output impedance, but I've emailed Scott & should have a reply soon.

I would investigate further interfacing the ATC's through the subwoofer... I haven't heard the Emes subwoofer, but I have heard their speakers - and they're no slouches. I would be surprised if they had put dodgy crossovers into their subs... but??

One thing to think about - if you run the speakers full range with a sub you're losing some of the benefits... when you filter off the bottom end going to the mains you increase the efficiency that they run at, meaning more sound with less strain.

When you say it sounded "terrible" - what do you mean by that??

Another option to think about is buying a crossover or making yourself a custom crossover.

[1] If you are sure that you wish to Y-split the Cranesong's outputs, you need to introduce a distribution amplifier for this purpose.

[2] Before doing this, get/make yourself a CD or signal generator with a range of tones each of which should last a decent length of time. Hook up your ATCs through the Emes as you originally had them. Run the output from the CD/signal generator straight to the input of the Emes. Listen carefully as each tone is reproduced. Since you feel that the Emes sounds correct and the ATCs not, focus your listening on those frequencies above the HP filter point you have selected on the Emes i.e. frequencies above 80/150Hz. Listen to 500Hz, 1kHz, 3kHz, 8kHz, 12kHz etc etc as reproduced by the ATCs under these circumstances. You should be quickly able to determine whether or not these tones sound correct. If not, the Emes must be faulty as you suspect. If yes, either there is no problem or it is more subtle.

[3] Check all your wiring to ensure that you have not introduced a phasing problem when adding the Emes into your setup.

In what way did it sound terrible?
Does the Emes have a level control? Because if you are using the Emes to split the audio band and pass the higher freqencies to the ATC's you'll need to level match to deal with the different sensitivites of the two speaker systems.
I can't image that the HEDD192 would have any trouble driving the Emes or indeed driving both the Emes and ATCs through a Y split.
Of course if you use a Y split you have two problems:
1 ] the ATCs will be getting the full range signal and cause general weirdness in interacting with the Emes sub
2 ] the Emes is a mono sub yeah? So it monos L and R, you can't do this with a simple Y cable.

I reckon go back to the convential setup and work on matching the sub to the ATCs and then describe more explicitly how it sounds.

I was hoping the Cranesong's outputs would have enough juice to happily drive a y-split arrangement.
I haven't read the specs on the Cranesong, but I don't doubt for a minute that it's outputs would have a low enough impedance to drive several inputs.

2) The Emes has the ability to bypass the HP filter on the line outputs, but this means there are preamps inbetween my D/A and ATCs.
How do you know this? And, who says it's a problem?

3) Cables are fine.
Have you used a phase-checker or carefully used a multimeter to verify this?

You haven't responded to any of the other posts asking you to clarify what you mean by bad sound, including mine where I suggest that you conduct some listening tests. There's not much point continuing this thread until you come back to us with some decent analysis of what you find is wrong with the sound.